An organic EL device is composed of layers of organic materials situated between two electrodes, which include a hole transporting layer (HTL), an emitting layer (EML) and an electron transporting layer (ETL). The basic mechanism of organic EL involves the injection of the carrier, transport, recombination of carriers and exciton formed to emit light. When an external voltage is applied to an organic EL device, electrons and holes are injected from a cathode and an anode, respectively, electrons will be injected from a cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from an anode into a HOMO (highest occupied molecular orbital). When the electrons recombine with holes in the emitting layer, excitons are formed and then emit light. When luminescent molecules absorb energy to achieve an excited state, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. 75% of the excitons form by recombination of electrons and holes to achieve a triplet excited state. Decay from triplet states is spin forbidden, thus, a fluorescence electroluminescent device has only 25% internal quantum efficiency. In contrast to fluorescence electroluminescent device, phosphorescent organic EL device make use of spin-orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and the internal quantum efficiency of electroluminescent devices from 25% to 100%. The spin-orbit interactions is finished by some heavy atom such as iridium, rhodium, platinum, palladium and the phosphorescent transition may be observed from an excited MLCT (metal to ligand charge transfer) state of organic metallic complexes.
Recently, a new type of fluorescent organic EL incorporating mechanism of thermally activated delayed fluorescence (TADF) has been developed by Adachi and coworkers is a promising way to obtain a high efficiency of exciton formation by converting spin-forbidden triplet excitons up to the singlet level by the mechanism of reverse intersystem crossing (RISC) by using a material having a small energy gap between the singlet level and the triplet level. However, further improvement in luminous efficiency of the organic EL device in a high current density region is still desired.
The phosphorescent organic EL utilizes both triplet and singlet excitons. Cause of longer lifetime and the diffusion length of triplet excitons compared to those of singlet excitons, the phosphorescent organic EL generally need an additional hole-blocking layer (HBL) between the emitting layer (EML) and the electron transporting layer (ETL) or the electron transporting layer with hole blocking ability instead of typical ETL. The purpose of the use of HBL or HBETL is to confine the recombination of injected holes and electrons and the relaxation of created excitons within the EML, hence the device's efficiency can be improved. To meet such roles, the hole blocking materials must have HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels suitable to block hole transport from the EML to the ETL and to pass electrons from the ETL to the EML, in addition, the good thermal and electrochemical stability of the materials are also needed.
For full-colored flat panel displays in AMOLED or OLED lighting panel the material used for the phosphorescent dopant for emitting layer are still unsatisfactory in half-life time, efficiency and driving voltage. These organic metallic complexes still have disadvantages for industrial practice use. The phosphorescent dopant with preferential in-plane (horizontal) emitting dipoles are beneficial to optical out-coupling of OLED, since the ratio of vertical emitting dipoles contributing little to external emission is reduced and the radiation pattern of a horizontal emitting dipole is in general more suitable for optical out-coupling. Therefore, an emitter with proper substituents can be helpful to enhance the ratio of horizontal emitting dipole in the emission layer of OLED. Meanwhile, the proper substituents around an emitter can effectively block nearby electrons and holes, so that electrons and holes can easily recombine in the emitter and the efficiency of OLED can be improved.
There continues to be a need for organic EL materials which is able to efficiently transport electrons or holes and block holes, with good thermal stability and more efficient EML materials for high emitting efficiency. According to the reasons described above, the present invention has the objective of resolving such problems of the prior-art and offering a light emitting device which is excellent in its thermal stability, high luminance efficiency, high luminance and long half-life time. The present invention disclose a novel material having general formula(1) or formula(2), used as phosphorescent emitting host, delayed fluorescent dopant, and hole blocking layer (HBL) have good charge carrier mobility and excellent operational durability can lower driving voltage and power consumption, increasing efficiency and half-life time of organic EL device.